This invention relates to a process for the production of perfluoroolefins by the thermal cracking of perfluorinated cyclobutanes prepared by Simons electrofluorination. More particularly perfluorocyclobutane is converted to tetrafluoroethylene and perfluoromethylperfluorocyclobutane is converted to a mixture of tetrafluoroethylene and hexafluoropropylene.
Tetrafluoroethylene (i.e., CF2xe2x95x90CF2 or TFE) and hexafluoropropylene (i.e., CF3CFxe2x95x90CF2 or HFP) are monomers used for the preparation of a variety of fluoropolymers. Typical fluoropolymer properties which enhance their utility include: excellent electrical insulation, resistance to attack by chemicals, wide service temperatures, from near absolute zero to about 300xc2x0 C., low coefficient of friction, anti-sticking, flame resistance and low smoke propagation. Both of the above fluoroolefins are commercially prepared by the high temperature pyrolysis of chlorodifluoromethane which itself is prepared from chloroform. There is a growing concern that chlorinated hydrocarbons pose a threat to the environment, especially the stratospheric ozone layer. Thus, a need now exists to develop fluoromonomer processes which are not chlorine based.
The production of TFE and HFP from precursors without chlorine has been reported. For example, one such process is based on the thermolysis of fluorinated cyclobutanes which can be prepared by electrochemical fluorination. European Patent Application No. 455,399 summarizes the known art describing electrochemical fluorination routes to octafluorocyclobutane and also discloses. . processes for the production of TFE and HFP by the pyrolysis of octafluorocyclobutane.
The use of induction heating to heat packing materials to cause chemical reactions of various types is known. For example, vinylidene fluoride is prepared by the reaction of methane and dichlorodifluoromethane in a reaction tube heated by induction heating and packed with high surface area porous carbon (see U.S Pat. No. 5,110,996, example 5).
This invention provides a process for producing a perfluoroolefin of the formula CF(R1f)xe2x95x90CF2, wherein R1f is selected from the group consisting of F and CF3. The process comprises (a) perfluorinating a cyclobutane starting material of the formula 
wherein R2f is selected from the group consisting of F and CF3 and each R1 is selected from H and CH3 provided that one R1 is H and the other R1 is H when R1f is F and is CH3 when R1f is CF3) by the Simons electrochemical fluorination process in an electrochemical cell in a solution of anhydrous liquid hydrogen fluoride under temperature and pressure conditions sufficient to replace all hydrogens in the cyclobutane starting material with fluorine; and (b) cracking the perfluorinated cyclobutane. In accordance with this invention, cracking can be accomplished by contacting the perfluorinated cyclobutane provided in (a) with carbon (e.g., graphite or activated carbon) or a conductive metal, which is heated by induction heating to a temperature sufficient to crack said perfluorinated cyclobutane.
The partially fluorinated cyclobutane starting materials of this invention can be prepared by thermal cyclodimerization of the olefin CHR1xe2x95x90CHR1 where R1 is as defined above (i.e., at least one must be H) with the olefin CF(R2f)xe2x95x90CF2 where R2f is as defined above. This includes the thermal cyclodimerization of ethylene with tetrafluoroethylene (TFE) to produce 1,1,2,2-tetrafluorocyclobutane, the thermal cyclodimerization of ethylene with hexafluoropropylene (HFP) to produce 1-trifluoromethyl-1,2,2-trifluorocyclobutane, the thermal cyclodimerization of propylene with TFE to produce 1,1,2,2-tetrafluoro-3-methylcyclobutane and the thermal cyclodimerization of propylene with HFP to produce 1-trifluoromethyl-1,2,2-trifluoro-3-methylcyclobutane. Typically, the mole ratio of ethylene or propylene to TFE or HFP is in the range of 0.3:1 to 50:1, preferably 5:1 to 20:1. The reactor used can be of any suitable type such as a tube and shell reactor, plug flow or turbulent flow reactor, or a continuous stirred tank reactor as described in EP 455,399. The thermal cycloaddition reaction can be achieved as described in U.S. Pat. No. 3,662,009 and U.S. Pat. No. 3,789,088. Typically, the cyclodimerization can be carried out at a temperature of from about 150xc2x0 C. to about 600xc2x0 C., preferably from about 300 to about 450xc2x0 C. Generally, the reaction pressure will be in the range of about 0.5 to 30 atmospheres, preferably in the range of about 2 to 10 atmospheres. Generally, the residence time is from about 1 second to about 10 minutes, preferably at the preferred temperatures and pressures, from about 1 to 2 minutes.
Of note are processes where in addition to CF(R1f)xe2x95x90CF2 product, CF(R2f)xe2x95x90CF2 is also produced and used for producing further partially fluorinated cyclobutane starting material by reaction with olefin of the formula CHR1xe2x95x90CHR1. Of particular note are processes where R2f is CF3, and where at least a portion of the hexafluoropropylene produced in (b) is reacted with CHR1xe2x95x90CHR1 to produce further cyclobutane starting material.
The process of electrolyzing liquid hydrogen fluoride containing an organic chemical which can be fluorinated, at an electrical potential insufficient to generate fluorine gas but sufficient to cause fluorination of the organic chemical is known. This electrochemical fluorination (ECF) is known as the Simons process. This technology has been described in U.S. Pat. No. 2,519,983, which is incorporated herein by reference and which contains a drawing of a Simons cell and its accessories.
The preferred starting materials for the ECF reaction are 1-trifluoromethyl-1,2,2-trifluorocyclobutane (I) and 1-trifluoromethyl-1,2,2-trifluoro-3-methylcyclobutane (II). The perfluorinated cyclobutane product selectivity is greater by about 5% to about 30% when compounds (I) and (II) are used. For both of these partially fluorinated cyclobutanes, the starting perfluoroolefin material for their preparation is hexafluoropropylene rather than tetrafluoroethylene.
The reactor effluent comprising perfluorinated cyclobutanes, hydrogen, hydrogen fluoride and other products, e.g., incompletely fluorinated cyclobutanes, can be separated by conventional techniques such as distillation and decantation.
After separation, the perfluorinated cyclobutanes can be cracked to a product mixture comprising tetrafluoroethylene and/or hexafluoropropylene by passage through an inductively heated reactor such as that described in International Publication No. WO 95/21126. The reactor is packed with a conductive metal, graphite, activated carbon or another carbon. A preferred carbon is a three dimensional matrix carbonaceous material prepared in a similar manner to that disclosed in U.S. Pat. No. 4,978,649.
The temperature within the reaction zone is between about 600xc2x0 C. to about 1200xc2x0 C. The residence time in the reactor is between about 30 milliseconds to about 3 seconds, preferably 300 milliseconds.
A diluent gas which is essentially inert under reaction conditions; such as, nitrogen and carbon dioxide, may be used if desired.
The reaction pressure is not critical and may be between about 50 kPa to about 130 kPa.
The hot exit gas leaving the reactor is rapidly cooled by quenching. Any known-art quenching procedures may be used. These include passing the hot exit gas over cold inert surfaces such as the outer surface of a water-cooled body. Alternatively, the hot exit gas may be mixed with a flow of inert cold fluid, e.g., water.
A particular advantage of the present invention is the low amount of perfluoroisobutylene found in the reactor products.